Innate spells, at most, are limited to nine, which is insufficient to meet the needs of a primordial soul wizard.
And most primordial soul wizards actually have just seven or eight innate spells... not many possess nine.
Therefore, primordial soul wizards need additional means to supplement their strength. Primordial soul spells are key.
Innate spells and primordial soul spells each have their advantages and disadvantages.
The former are more convenient to use, resembling the divine abilities of transcendent creatures. With just a thought, they can be activated, consuming relatively little energy.
To draw an imperfect analogy,
in most cases, innate spells could be seen as "ordinary attacks", prized for their instant casting and no mana cost, suitable for prolonged combat.
However, innate spells are, after all, created during the period when one is an intermediate wizard.
Their power, of course, continually upgrades as the wizard's realm improves.
